The subject matter disclosed herein relates to a transfer switch for automatically switching between a primary power source and a secondary power source. More specifically, a transfer switch for automatically switching between an alternative energy source and the utility grid is disclosed.
Historically, it has been known to provide a transfer switch to automatically transfer a load from the utility grid to a backup generator in the event the utility grid was not operating properly. For example, the voltage present on the utility grid is monitored, and if the voltage drops below a preset value, the transfer switch switches to a backup power source, such as a generator. Further, the automatic transfer switch may provide a signal to the generator to start the engine and wait until the generator is operating normally before connecting the load to the generator.
With the development of alternative energy sources and their associated power conversion equipment, it may be desirable to provide a power source other than the utility grid as the primary power source. An alternative energy source, such as a photovoltaic array or a wind turbine, may be selected. However, the power supply from such alternative sources is variable, dependent on the amount of sunlight or wind present at a given time. As a result, alternative energy sources are often utilized in combination with an energy storage device. Because the energy storage device stores and supplies a DC voltage, an inverter is provided which converts the DC voltage to an AC voltage suitable for connection to the load.
However, a traditional automatic transfer switch is not well suited for an application in which the primary energy source is an alternative energy source and the secondary energy source is the utility grid. As previously discussed, a typical automatic transfer switch monitors the voltage level of the primary energy source and switches to the secondary source when the voltage level drops below a minimum value. However, an inverter supplying the AC voltage will attempt to regulate the output voltage to maintain the nominal AC voltage, for example 120 VAC. If the alternative energy source is not producing sufficient power to supply the load and the battery has been drained, the inverter will experience a fault condition and shut down. Manual intervention is required to reset the inverter, defeating the purpose of an automatic transfer switch. Further, due to the variable energy production, it is expected that more frequent switching from an alternative energy source to the utility grid would be required than when the utility grid is the primary energy source.
Thus, it would be desirable to provide an automatic transfer switch suitable for use with an alternative energy source as the primary energy source and the utility grid as a secondary source.